MEMBRANE PERMEABILTY
Figure 1. A higher volume of nitrogen gas is able to pass through PIM-1 into the orange balloon compared with the membrane on the right, connected to the pink balloon. Source: Kyoto University iCeMS
Led by Easan Sivaniah, an associate professor at
Kyoto University’s Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto, Japan, the team consists of researchers from iCeMS and the
University of Cambridge, Cambridge, U.K.The membrane, referred to as PIM-1, can trap gases in a network of channels and cavities less than 2 nm in diameter. However, PIM-1’s starting selectivity and stability is weak. To improve the membrane, Sivaniah’s team heated PIM-1 at temperatures ranging from 120° to 450°C in the presence of oxygen. This reinforced the strength of channels while controlling the membrane’s permeability, which increased selectivity. After completing the thermal oxidation process, PIM-1 was found to be twice as selective for carbon dioxide while allowing air to pass through it 100 times faster compared with commercially available polymers. More details appear in a recent article in the journal
Nature Communications.“In this work, we demonstrate a simple process of thermal oxidative crosslinking of independent rigid polymer chains to covalently crosslinked polymer networks with significantly enhanced molecular-sieving selectivity and exceptional gas separation performance. The next step … is applying the thermal oxidation method to thin film composite membranes (TFCs) of PIM-1 to achieve high performance, high selectivity and high flux, gas separation membranes. In this study, we have demonstrated that the oxidative crosslinking is effective for PIM-1 polymer thin films coated on silicon or glass substrates. Similarly, we expect that thin films coated on thermally stable porous membrane supports can be exposed to heat treatment at controlled atmosphere, potentially allowing the scale up of our process. At the same time, our group is working on utilizing thermal oxidative cross linking method to the other new types of microporous polymer materials,” Sivaniah elaborates.The membrane is affordable and long-lasting. Savianah explains: “PIM-1 undergoes physical aging, which is the gradual relaxation of non-equilibrium excess free volume in glassy polymers. Thermal oxidative crosslinking PIM-1 was undertaken in an attempt to increase the physical and chemical stabilities in the final membranes. Crosslinking prevents the membranes from plasticizing in the presence of aggressive agents as well as promotes thermal stability. The stabilized membrane has a lower permeability than pure PIM-1, but it also keeps its selective properties as feed pressure increases, thus assuring final integrity and performance reliability of the membranes, which are important factors for industrial applications.”Key challenges the team must address include control of crosslinking and industrial scaleup. Sivianah also says processing parameters in making defect-free thin film and proper heat resistance support are important issues to solve for preparing TFC membranes. “In addition to large-scale membrane production problems, some engineering issues of the membrane material should be solved before performing in pilot-plant scale. Membrane systems engineering are depending on various aspects, such as membrane module configuration and units arrangement to work in optimal operation conditions.” “PIM-1 is still a relatively new material; it is polymeric and it is solution-processable, so in principle there are no generic barriers to fabricating the membranes but each polymer chemistry has its individual issues that will arise and which must be dealt with,” he adds.
